Gas processing and cleanup is a critical operation in the chemical industry. Several industrial processes utilize gases that need to be cleaned and the various contaminants (such as H2S, SO2, COS, HCl, NH3, etc.) removed prior to their use. In addition to removal of contaminants, the gas composition may also need to be adjusted to meet process requirements for H2, CO and/or CO2 content.
One of the process gases that are used heavily for production of chemicals and power is synthesis gas or “syngas”. Syngas is produced from partial combustion of organic feedstocks (coal, petcoke, biomass, oil) and consists primarily of CO and H2. Syngas often contains contaminants (including H2S, COS) depending on the starting raw material. The H2S and COS in the syngas can de-activate the catalysts used in the downstream processes and need to be removed to very low levels. In case of power production, the sulfur species can oxidize and produce SO2 during combustion which is regulated by the Environmental Protection Agency (EPA) to reduce acid rain. As appreciated by persons skilled in the art, other process gases likewise often require cleanup, one further example being natural gas.
Several technologies have been developed to meet this need. Most of the technologies use a solvent-based approach where the gas species that need to be removed are absorbed in the solvent under pressure at ambient or sub-ambient temperatures, and the solvent is later regenerated by either flashing the solvent (reducing the pressure) or by use of thermal energy (heating the solvent). Examples of such processes include the SELEXOL® process by Dow Chemicals (licensed to UOP) which uses a mixture of dimethyl ethers of polyethylene glycol (DEPG), RECTISOL® by The Linde Group and Lurgi AG which uses methanol as the solvent, amines (such as MDEA, MEA, DEA etc.) as well as activated MDEA by BASF Corporation, Shell Corporation, and UOP. These solvent-based removal processes are typically referred to as acid gas removal (AGR) processes.
The H2S, COS, and CO2 are soluble in the different solvents to varying degrees, and the solvent-based processes are quite complex and are designed to separate out the H2S and COS into separate streams. H2S/COS stream is used further downstream, either for sulfur recovery or production of sulfuric acid. The CO2 stream can be used in enhanced oil recovery (EOR) or stored in geological aquifers or can be used to produce value-added products such as algae, among other uses.
Chemical applications of syngas, such as methanol conversion or Fischer-Tropsch conversion to fuels, typically require the sulfur levels in the syngas to be very low, such as less than 100 ppbv. This ultra-low sulfur requirement is difficult for most AGR processes to achieve. It would be desirable to be able to decouple the process of removing sulfur compounds from the process of removing CO2 in a way that would optimize the removal of both sulfur compounds and CO2, whereby sulfur compounds could be reduced to lower levels in the process gas, and higher levels of purity of the sulfur compounds and CO2 could be achieved, than would be possible from performing any of the conventional AGR processes alone. Such decoupling could enable a number of these AGR technologies to be used effectively in process gas-to-chemicals or fuels applications where these AGR technologies cannot be used currently and/or could enable a reduction in capital costs and/or utility costs.
Syngas is the starting material for production of a variety of chemicals. Syngas can also be used for power production in a gas turbine. Syngas can also be used to produce H2, by converting the CO to H2 via the water-gas-shift (WGS) process and removing the CO2 in the gas stream and purifying the treated gas using a pressure swing adsorption (PSA) or a membrane process. The H2 to CO ratio of the process gas needs to be carefully adjusted to meet the downstream applications demand.
The WGS reaction is utilized to shift carbon monoxide (CO) to carbon dioxide (CO2) and diatomic hydrogen gas (H2) by reacting the CO with steam over a catalyst bed. WGS is an industrially important process utilized to increase the H2/CO ratio to meet the downstream process requirements of a particular application. For example, WGS finds applications in pre-combustion CO2 capture where a fuel is partially oxidized to produce synthesis gas (or “syngas,” predominantly consisting of CO+H2). This syngas is shifted to maximize the H2 and CO2 concentrations, and CO2 is removed prior to combustion of the H2-rich clean gas in turbines for generating electricity. WGS also finds widespread applications in chemicals production where the H2/CO ratio needs to be adjusted as per the process requirements. For example, the synthesis of methanol (CH3OH), CO+2H2→CH3OH, requires the H2/CO ratio to be 2.
In traditional AGR processes such as the RECTISOL® and SELEXOL® processes, the WGS is done upstream of the AGR process and is called a “sour gas shift.” The gas to be shifted contains sulfur (as hydrogen sulfide (H2S) and carbonyl sulfide (COS)) and requires an expensive catalyst that is sulfur tolerant and promotes the shift reaction in the presence of H2S and COS. Examples of sulfur tolerant shift catalysts include cobalt-molybdenum (Co—Mo) and nickel-molybdenum (Ni—Mo). When the shift is carried out downstream of the AGR, it is termed as “sweet gas shift” and does not require a sulfur tolerant catalyst. The sweet shift catalysts are less expensive than the sulfur-tolerant sour gas shift catalyst. Thus, it would be desirable to be able to decouple the process of removing sulfur compounds from the process of removing CO2 so as to facilitate implementation of the WGS downstream of the sulfur removal process. This may enable better control over the H2/CO ratio and/or removal of CO2, as well as the use of the less expensive sweet shift catalysts.